1. Field of the Invention
Aspects of the present invention relate to a membrane electrode assembly for a fuel cell as well as a fuel cell employing the same; and more particularly, to a membrane electrode assembly for a fuel cell in which power generating performance is improved by adjusting the doping level of acid in a polymer electrolyte membrane, as well as a fuel cell using the same.
2. Description of the Related Art
Ion conductors, through which ions can move when voltage is applied, are widely used in electrochemical devices, such as batteries, electrochemical sensors, and the like.
For example, proton conductors, which have stable proton conductivity in a dry environment (non-humidified or a relative humidity of 50% or less) at an operating temperature of 100 to 300° C., are preferably used in fuel cells in terms of power generating efficiency, system efficiency, and long-term durability of the components of the electrochemical devices.
The development of the conventional solid polymer fuel cell has been reviewed, considering the requirements. However, a solid polymer fuel cell containing a perfluoro carbon sulfonic acid membrane as a polymer electrolyte membrane cannot generate sufficient electricity in a dry environment (non-humidified or a relative humidity of 50% or less) at an operating temperature of 100 to 300° C.
In addition, there are fuel cells using a polymer electrolyte membrane containing a proton conducting agent, a silica dispersing membrane, an inorganic-organic composite membrane, a grafted membrane doped with phosphoric acid, or an ionic liquid composite membrane. Also, U.S. Pat. No. 5,525,436 discloses a solid polymer electrolyte membrane composed of polybenzimidazole (PBI) doped with a strong acid, such as phosphoric acid.
The polymer of the electrolyte membrane described in U.S. Pat. No. 5,525,436 is doped with about 350 mole % of phosphoric acid in order to obtain high proton conductivity, while the electrodes are not separately doped with an acid. When electrodes of the membrane electrode assembly employing the polymer electrolyte membrane are doped with more acid, the polymer electrolyte membrane is decomposed by the excess phosphoric acid so that crossover of reaction gases occurs.
Furthermore, a polymer electrolyte membrane doped with about 350 mole % of phosphoric acid includes excess phosphoric acid even taking into account the fact that a portion of the phosphoric acid contributes to proton conductivity of the polymer electrolyte membrane, and the excess phosphoric acid spreads to the electrodes and is doped into the electrodes so that the doping amount of phosphoric acid in the electrodes cannot be adjusted precisely.